Saburo (サブロー ,Saburō ), also known as 623 (Mutsumi,623) or Mutsumi Hojo (北城睦実,Hōjō Mutsumi) in the manga is a main character in the Keroro Gunso series.

Saburo also appears frequently in Seven of Seven as the radio DJ "623". Mine Yoshizaki, the mangaka of Keroro Gunso, did character draft work on Seven of Seven - Saburo is one of several characters who cameo on the show.

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Invaders from a race of human-like super intelligent aliens called N326ans were about to leave Earth after a failed operation, when they were approached by Mutsumi Hojo. In spite of being a child, he was was able to see them and immediately recognized they weren't humans. Revealing that he had no father or mother anymore, he asked to be taken by them. One of the members of the invasion force decided to accept him and raise him as his own son. Years later, Mutsumi would return to Earth, curious about the Keronians' activity there.[1]

Although this story only was published after the anime ended, the name taken by Mutsumi there (Saburo/326) was foreshadowing for the name of the race of his adoptive parent and, after the first few episodes, he was drawn with colored pupils, like most human-like alien characters.

Eventually, Mutsumi found Kururu after the Keroro Platoon had split once descending to Earth for direct invasion. Kururu was curious about the human able to see through the Anti-Barrier and they fought against each other, only to quickly become partners. Kururu gave him the Reality Pen, which makes any drawing on a flat surface real.[2][3]

Mutsumi is a high school student in the manga, while Saburo is a middle school student in the anime. He has an I.Q. of 350 and is Kururu's partner, due to having similar wavelengths. He is the least-active main character of the series, but plays a fairly significant role as Kururu's partner/friend and Natsumi's crush. In the anime, Saburo's identity as Kururu's collaborator is at first a secret (though it's quite obvious), whereas in the manga it comes almost immediately to light.

He skips class a lot and is rumored at school to be a member of the "Men in Black", though that doesn't seem to affect Natsumi's high opinion of him. Saburo is excellent in sports, academics, poetry, art, and philosophy, which is why he has a hard time fitting in with his classmates. There are even rumors about him being an alien[4]. He hosts a late-night radio program which Natsumi compulsively listens to.

Saburo's name is based on Japanese wordplay (Saburo means 326, and Mutsumi means 623). Saburo's wordplay name, 326, also first appears in episode 112 on the Reality Pen.

Saburo in his first appearance in Volume 2, Ch. 19

In the anime, his identity as 623 is only known as the radio host, and his identity is kept secret to the public, while he uses his identity as Saburo in public life. While the 623/Saburo connection is hinted at several times in the anime, it isn't until episode 163 that this is officially revealed.

In the anime, he uses his free time to cross into alternate dimensions using the Reality Pen to create gates, but that is a secret that only Fuyuki Hinata knows[4]. In the manga, although he hasn't been shown going into other dimensions for leisure, he was able to break into a sub-dimension created by his father and was shown to be able to float even without using the reality pen.[1]

In the anime, Saburo is a fairly tall character with tan skin, royal blue eyes and short, spiky silver hair. He usually wears a white bucket hat pinned with a yellow spiral, representing Kururu's symbol.

In the Flash series, Saburo's skin is a lighter tan and his eyes appear to be a litter shade of blue. He is depicted with small white wings, much like his manga design.

During his introduction in the second volume, Saburo is shown as calm, cool, and mysterious. However, when he reveals Kururu to the platoon, he shifts to a happier personality. He even accepts Tamama's ideas about Coke from the faucets with joy.

He turns out to be a loyal and gentle, but solitary guy - if he's aware of Natsumi's crush, he hasn't shown it, though he does take pain in reading out her postcards when she sends them in to his radio show.

Saburo is the least participant human who is aligned with the Keroro Platoon, and has made the least appearances. However, this does not mean he is lazy or weak, in fact, quite the contrary. He volunteers on plots that interest him, frequently jokes around with the platoon members and is a very good comrade when in serious battles. In his appearances, he gives a great deal of support with the help of his virtual reality pen, which can create unimaginably powerful weapons and fascinating objects if he puts his imagination into it and draws it on paper, and with a quick and cunning mind Saburo has. This is pretty much the perfect weapon for him. He also maintains good relationships with his comrades, thanks to his easygoing nature and is never seen to be completely enraged or angry.

Kururu is the first member of the Keroro platoon that Saburo has met, but their beginning isn't exactly much for a start of a companionship. Kururu and Saburo immediately engage in fight, which after a surprising turn of events, Saburo saves Kururu's life while he gives the boy his virtual reality pen in return (anime). Since they're both misunderstood geniuses, they are capable of getting along with each other, and Saburo seems to be the only one who is shown to care about Kururu as a true friend. Saburo helps Kururu and his comrades on broadcasting-based plots, since Saburo is the anonymous (at least in the anime) 623, a popular radio DJ whom Natsumi is a big fan of. In the episode 229, when Kururu is alone, he says that he "can not let [their] bonds fade" and blows the whole invasion budget just to help Saburo. This hints that Kururu cares about Saburo as a friend as well.

Natsumi has been hinted to have a crush on Saburo ever since the first episode of the anime, but played no further part in some later episodes but his anonymous (at least in the anime) alter-ego is introduced as Natsumi's favorite radio DJ. Then later, in episode 9, he is introduced as the human partner of Kururu, the most secretive, sly and somewhat irritating member of the Keroro Platoon. Ever since then, Natsumi and Saburo have had more closer encounters before, but almost all of them end up as Natsumi being very embarrassed. Saburo thinks of her as a friend but hasn't showed any other feelings yet, while Natsumi desperately wants to confess to him. Sometimes Saburo acts like he may not notice, and sometimes he knows Natsumi Hinata likes him.

Koyuki is one of his close friends, Saburo connects to her and they both support each other, often she connects with him than any other guy, they like to hang out with each other mostly when he is associating with Dororo. Koyuki sees him as a dear friend, who she can trust. In the manga, he gives her nicknames such a cutiepie and ninja girl, he is flirtatious around her but also expresses a caring/friendly emotion when she is present. When there is danger, they work as a team along with the Hinatas, and Momoka, she has strong feelings for him as he does for her, in the Anime they have a similar connection and have a close relationship.

Originally, Fuyuki feared Saburo, much like the other students at school and when Natsumi stated he was invited over to their house, Fuyuki feared he knew about Keroro and wanted to capture him and the others. Despite the rough start, the two have become close friends. Saburo causally talks to Fuyuki on a daily basis, even hugging him upon greeting.

Saburo appears for the first time in episode 6 as the late-night radio host 623. From there onward, 623's voice is usually featured at least once per episode on Nana's radio, and often just before the ending credits as well, toting a poem written by the DJ. Saburo's poems were often seen in the early Keroro stories as well; in the manga, he is a self-titled philosopher with an easygoing nature. 623 met Nana when she came to his radio station, and he appears again in person later in the series.

The most notable difference between the Seven of Seven and Keroro Gunso versions of Saburo is the color of his hair; in the former, it is an electric blue, and in the latter, a silvery-grey. He's voiced by Akira Ishida in both shows, though in Seven of Seven his voice is credited as simply "623, by 623." In the English dub, he is voiced by Sebastian Arcelus.